Maybe that was the right call. The ultimate outsider stayed on the outside and on the campaign trail, holding rallies and building toward another big expected haul of delegates in next Tuesday’s northeastern primaries. Here with the insiders, RNC members from all 56 states and territories, plus state party leaders, there was an unmistakable sense that a Trump nomination, if not inevitable, is now the probable outcome at the Republican convention in July.

And they’re OK with that. For some, that conclusion comes with a tinge of resignation. For others, there was a “look on the bright side” element to their analysis. Look at the massive turnout in the GOP primaries and all the problems likely Democratic nominee Hillary Clinton has, they note – starting with questions about her honesty and the rise of democratic socialist Bernie Sanders, with his fundraising and massive crowds. And look at Trump’s ability to electrify supporters, a skill Mrs. Clinton lacks, they add.

But Trump still has work ahead. He may well finish the primaries in June without enough convention delegates to lock up on the nomination. In that case, “he’ll still have homework to do,” says Jonathan Barnett, Republican national committeeman from Arkansas.

“He’s going to have to get more friendly and start being nicer to the party people, nicer to the opposing candidates and the campaigns,” says Mr. Barnett, who is pledged to Trump on the first ballot. “If you can’t bring people together, you can’t build a coalition, and if you can’t build a coalition you can’t lead.”

Barnett attended the Trump reception, where Dr. Carson spoke and where Trump’s new top campaign advisers, Paul Manafort and Rick Wiley, made the case for why Trump would be the strongest general election candidate.

Press were not allowed in the room, but others who were there said the campaign laid out a map that showed Trump competitive in blue states such as New York, New Jersey, Pennsylvania, and Illinois – an assertion that party veterans greeted skeptically.

Trump, a team player?

But first things first: For Trump to win enough delegates at the convention, he’ll need party regulars on his side. Ditto if he wins the nomination; the man who answers to no one will need the very insiders he disdains to work with him and for him. For starters, that means dropping the rhetoric trashing the RNC and what he calls the “rigged” nomination process.

“We’ve talked about that [with Trump] on a number of occasions,” Carson told reporters before the reception. “He’s trying to moderate, he’s getting better. It doesn’t happen overnight.”

RNC veteran Henry Barbour says he gets what Trump is doing, even if he disagrees – and he’s now willing to vote for him in November.

“I’ve now gotten to the point where I’m OK” with a Trump nomination, says Mr. Barbour, the RNC committeeman from Mississippi and co-author of the RNC’s 2013 report on how to expand the party’s appeal and retake the presidency. “I’m going to support our nominee. That’s not to say I can’t change my mind, but I think I’m typical. The guy irks me at times, but I get that he’s doing his populist gig, and he’s trying to win.”

Still, Trump’s trash talk of the RNC “hurts the institution,” adds Barbour, who backed first Rick Perry, then Marco Rubio, and is now uncommitted. “I think he would do better to talk about bringing Republicans together on the issues that unite us.”

And that, he says, would go a long way toward softening the opposition to Trump among the GOP elite as well as rank-and-file voters. “If Donald Trump is our nominee,” he says, “I think a lot of people who today say I’d never vote for Trump will get over that in September and October when they think about what a Hillary Clinton would mean to our country.”

Other RNC members are less hesitant about backing Trump in November.

What insiders are saying

“I feel great,” says Ron Kaufman, the RNC committeeman from Massachusetts who originally backed Jeb Bush. “I’m OK with whoever the nominee is going to be.”

Mr. Kaufman cites as positives the historically high turnout in Republican primaries and huge viewership for the debates. “Listen, the party that has problems is the Democrats,” he says. “Clinton is the presumptive nominee, and yet dead-man walking Bernie Sanders has outraised her for the fourth month in a row by tens of millions of dollars.”

Kaufman acknowledges that the highly watched debates haven’t always helped the party’s image. “We can’t control the candidates,” he says.

Another RNC member, Paul Reynolds of Alabama, says the content of the debates became problematic a few times, and suggests instituting a penalty system for rhetoric that goes over the line. But he acknowledges that “you get into a slippery slope when you try to do anything on the content.”

Mr. Reynolds, a bound delegate for Trump, makes clear that he’s not anti-Ted Cruz. And he says the reception with Trump’s advisers was cordial. “It was bridge-building, taking care of some frayed nerves and a few egos that had gotten bruised,” he says.

As for being a Trump delegate, he adds, “it doesn’t bother me at all. We just want to get everything over and get into the general election so we can start focusing.”

Of course, not everyone at The Diplomat seaside resort in Hollywood, where the RNC convened, was ready to give in to Trump-mentum. Advisers to both Senator Cruz and Ohio Gov. John Kasich roamed the corridors and meeting rooms, wooing RNC members and state party chairs – all of whom are also convention delegates. Cruz and Governor Kasich both also appeared in person, making their case to the party elite and to the press. Electability and more faithful adherence to GOP principles were top arguments.

“I think we made a good impression on most,” says Saul Anuzis, delegate wrangler for Cruz.

And it was clear, in interviews with these most active of Republican activists, that if Trump can’t win on the first ballot, it will be very difficult, if not impossible, for him to win on the second or third.

Barnett of Arkansas originally backed his state’s former governor, Mike Huckabee, for president, but is a Trump delegate, at least for the first ballot.

And what about after that?

“As of now, I’d vote for Trump on the second ballot, most likely – unless I had a reason not to,” Barnett says. “And that’s up to him.”